Particularly in integrated circuit structures the active components of radio devices, such as amplifiers and mixers, are often realized as differential components, which means that the active component has two inputs and two outputs, whereby the input signal is a variable voltage between the two inputs and the output signal is a variable voltage between the two outputs. An alternative to the differential component is a single input and single output component where the input signal is a variable voltage between the input and a fixed ground potential, and the output signal is a variable voltage between the output and a fixed ground potential. An advantage of the differential structure is particularly that there is less variations in the component's performance caused by the manufacturing process. In the signal propagation direction there can be for instance a balun circuit, an amplifier, a filter or a mixer in front of the differential active component.
FIG. 1 shows a typical prior art differential amplifier 100, which has inputs RF+ and RF- provided with decoupling capacitors, and outputs OUT1 and OUT2. Two transistors Q1 and Q2 act as the amplifying components; the positive operating voltage Vcc is supplied to their collectors via the collector resistors RC and their emitters are connected via the emitter resistors RE and a constant current source Idiff to the ground potential. In addition to the input signal also a bias voltage Vb is supplied to both transistor bases via the biasing resistors Rb in order to bias the transistors Q1 and Q2 to the correct operating point. The output signal is taken at the collectors of the transistors Q1 and Q2.
In the operation of an amplifier according to the FIG. 1 a problem is created by its input impedance. In the signal propagation direction there is most commonly a filter (not shown in the figure) in front of the amplifier, whereby the filter can directly have a differential output, or its non-differential output can be duplicated with a so called balun before it is connected to the inputs of the amplifier. The frequency response of the filter depends on the input impedance of that component to which the signal is supplied from the filter. The input impedance of the amplifier shown in figure is as such very high, i.e. of the order of megaohms. In order to have a correct function of the circuit arrangement formed by the filter and the amplifier in series with it the input impedance of the amplifier must be adjusted to a value, which is a few tens or at most hundreds of ohms. 50 ohm has become a kind of a standard value for the impedances between RF components, but depending on the details of the filter structure a suitable value of the input impedance can also be for instance 100 or 200 ohms.
A simple way to adjust the input impedance of the amplifier according to FIG. 1 as 200 ohms is to select 100 ohms as the value of both biasing resistors Rb. Another common way to arrange the input impedance of the amplifier according to the FIG. 1 is that a resistor with a resistance equalling the desired input impedance is connected between the inputs RF+ and RF-. A disadvantage of these solutions is that they impair the noise characteristics of the amplifier.
FIG. 2 shows a more advanced solution, the so called collector feedback. In this solution the signal taken from the collectors of both amplifier transistors Q1 and Q2 is supplied to the bases of the additional transistors Q3 and Q4, and the signal to the amplifier outputs OUT1 and OUT2 is taken at the emitters of the transistors Q3 and Q4. The transistors Q3 and Q4 are supplied with the operating voltage from the common operating voltage source Vcc, and the emitters of both transistors are connected via an own constant current source Idiff3 and Idiff4 to the ground potential. From the emitter of the transistor Q3 there is a connection via the feedback resistor Rfb and a decoupling capacitor to the base of the transistor Q2, and from the emitter of the transistor Q4 there is a similar connection to the base of the transistor Q1. The input impedance can be affected by selecting the values of the feedback resistors Rfb in a suitable way. The circuit arrangement according to the FIG. 2 has generally a high gain and relatively good noise characteristics, but a poor reverse isolation. The last mentioned disadvantageous characteristic means that if undesired oscillations are coupled to the output of the amplifier, for instance from a mixer (not shown in the figure) in series with the amplifier, these oscillations will propagate relatively easily through the amplifier in a direction opposite to that of the signal, and thus these oscillations can be coupled from the input of the amplifier to other parts of the radio device causing interference there (for instance in the antenna).
FIG. 3 shows another prior art way to adjust the input impedance of an amplifier. The circuit arrangement is in other respects similar to that of FIG. 1, but the input signal is not supplied from the inputs RF+ and RF- to the bases of the transistors Q1 and Q2 but to the emitters, and the bases of the transistors Q1 and Q2 are interconnected, whereby the biasing can be made with one biasing resistor Rb. Regarding the radio frequencies the bases of the transistors Q1 and Q2 are connected via the decoupling capacitor to the ground potential. The input impedance is mainly determined by the value of the series resistances Rin. The circuit according to the FIG. 3 is suited only for very low values of the input impedances, because its gain G will always be lower than the ratio of the collector resistances RC to the serial resistances Rin, or G&lt;RC/Rin. At greater values of the input impedance the circuit arrangement according to the FIG. 3 does not sufficiently amplify the signal.